Addition–elimination versus Tishchenko reaction in the gas phase

Gas phase reactions of the substituted phenide ions with methyl formate have been studied. It was found that the results of these reactions depend mainly on the basicity of the phenide ion, which is related to the presence of the electron‐accepting or electron‐donating substituents in the benzene ring. It was shown that the phenide ions substituted with electron‐withdrawing groups react with methyl formate in the gas phase in a two‐step reaction. The first step that proceeds according to the typical addition–elimination mechanism results in the formation of the anion of the respective benzaldehyde derivative with the negative charge located either in the aldehyde group (acyl anion) or in the benzene ring (phenide anion) in position ortho to an aldehyde moiety. In the second step, the preliminary‐formed anion reacts with the second molecule of methyl formate yielding formally product of the second addition–elimination reaction. Theoretical calculations as well as collision induced dissociation spectra of the model compounds suggest that this reaction proceeds according to the Tishchenko reaction mechanism yielding the respective phthalide anion. According to our knowledge, this is the first example of the Tishchenko‐type reaction in the gas phase. Copyright © 2014 John Wiley & Sons, Ltd.     

Substitution reactions of gaseous ions in a three‐dimensional quadrupole ion trap

Substitution reactions between gaseous ions and neutral substrate molecules are of ongoing high interest. To investigate these processes in a qualitative and quantitative manner, we have constructed a device, with which a defined amount of a volatile substrate can be mixed with a defined amount of helium gas and added into a three‐dimensional quadrupole ion trap. From the known inner volume of the device, the known ratio n_substrate:n_He of the mixture, and the determined absolute partial pressure of helium in the ion trap, we can derive the partial pressure of the substrate in the ion trap and, thus, convert the directly observable pseudo–first‐order rate constants of the substitution reactions into absolute bimolecular rate constants. We have tested the device by investigating a series of S_N2 reactions of Br ^? and CF₃CH₂O ^? anions as well as ligand exchange reactions of ligated Na⁺ cations. As the obtained results suggest, the described device makes it possible to determine the bimolecular rate constants of substitution reactions as well as other ion‐molecule reactions with satisfactory accuracy and reliability.     

Theoretical study of the gas‐phase ion pairs SN2 reactions of LiX with CH3SY (X,Y = F,Cl, Br,I)

The gas‐phase ion pair S_N2 reactions at saturated sulfur LiX + CH₃SY → CH₃SX + LiY (X, Y = F, Cl, Br, I) are investigated using the CCSD(T) calculations. The calculated results show that the reactions LiX + CH₃SY are exothermic only when the nucleophile is a heavier lithium halide. Central barrier heights are found to depend primarily on the identity of nucleophile LiX, decreasing in the order LiF > LiCl > LiBr > LiI. Another interesting feature of the ion pair reactions at sulfur is the good correlation between the reaction barriers with geometrical looseness of Li? X and S? Y bonds in the transition state structures. The data for the reaction barriers show good agreement with the prediction of the Marcus equation and its modification. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Study of gas‐phase reactions of NO2+ with aromatic compounds using proton transfer reaction time‐of‐flight mass spectrometry

The study of ion chemistry involving the NO₂⁺ is currently the focus of considerable fundamental interest and is relevant in diverse fields ranging from mechanistic organic chemistry to atmospheric chemistry. A very intense source of NO₂⁺ was generated by injecting the products from the dielectric barrier discharge of a nitrogen and oxygen mixture upstream into the drift tube of a proton transfer reaction time‐of‐flight mass spectrometry (PTR‐TOF‐MS) apparatus with H₃O⁺ as the reagent ion. The NO₂⁺ intensity is controllable and related to the dielectric barrier discharge operation conditions and ratio of oxygen to nitrogen. The purity of NO₂⁺ can reach more than 99% after optimization. Using NO₂⁺ as the chemical reagent ion, the gas‐phase reactions of NO₂⁺ with 11 aromatic compounds were studied by PTR‐TOF‐MS. The reaction rate coefficients for these reactions were measured, and the product ions and their formation mechanisms were analyzed. All the samples reacted with NO₂⁺ rapidly with reaction rate coefficients being close to the corresponding capture ones. In addition to electron transfer producing [M]⁺, oxygen ion transfer forming [MO]⁺, and 3‐body association forming [M·NO₂]⁺, a new product ion [M−C]⁺ was also formed owing to the loss of C═O from [MO]⁺.This work not only developed a new chemical reagent ion NO₂⁺ based on PTR‐MS but also provided significant interesting fundamental data on reactions involving aromatic compounds, which will probably broaden the applications of PTR‐MS to measure these compounds in the atmosphere in real time.     

Mechanism for the gas‐phase reaction between OH and 3‐methylfuran: A theoretical study

The mechanism for the OH + 3‐methylfuran reaction has been studied via ab initio calculations to investigate various reaction pathways on the doublet potential energy surface. Optimizations of the reactants, products, intermediates, and transition structures are conducted using the MP2 level of theory with the 6‐311G(d,p) basis set. The single‐point electronic energy of each optimized geometry is refined with G3MP2 and G3MP2B3 calculations. The theoretical study suggests that the OH + 3‐methylfuran reaction is dominated by the formation of HC(O)CH?C(CH₃)CHOH (P7) and CH(OH)CH?C(CH₃)C(O)H (P9), formed from two low‐lying adducts, IM1 and IM2. The direct hydrogen abstraction pathways and the S_N2 reaction may play a minor or negligible role in the overall reaction of OH with 3‐methylfuran. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Amine‐Appended Hierarchical Ca‐A Zeolite for Enhancing CO2/CH4 Selectivity of Mixed‐Matrix Membranes

An amine‐appended hierarchical Ca‐A zeolite that can selectively capture CO₂ was synthesized and incorporated into inexpensive membrane polymers, in particular polyethylene oxide and Matrimid, to design mixed‐matrix membranes with high CO₂/CH₄ selectivities. Binary mixture permeation testing reveals that amine‐appended mesoporous Ca‐A is highly effective in improving CO₂/CH₄ selectivity of polymeric membranes. In particular, the CO₂/CH₄ selectivity of the polyethylene oxide membrane increases from 15 to 23 by incorporating 20 wt % amine‐appended Ca‐A zeolite. Furthermore, the formation of filler/polymer interfacial defects, which is typically found in glassy polymer‐zeolite pairs, is inhibited owing to the interaction between the amine groups on the external surface of zeolites and polymer chains. Our results suggest that the amine‐appended hierarchial Ca‐A, which was utilized in membrane fabrication for the first time, is a good filler material for fabricating a CO₂‐selective mixed‐matrix membrane with defect‐free morphology.     

From the mobile proton to wandering hydride ion: mechanistic aspects of gas‐phase ion chemistry

Structural characterization of molecular species by mass spectrometry supposes the knowledge of the type of ions generated and the mechanism by which they dissociate. In this context, a need for a rationalization of electrospray ionization(+)(?) mass spectra of small molecules has been recently expressed. Similarly, at the other end of the mass scale, efforts are currently made to interpret the major fragmentation processes of protonated and deprotonated peptides and their reduced forms produced in electron capture or electron transfer experiments. Most fragmentation processes of molecular and pseudo‐molecular ions produced in the ion source of a mass spectrometer may be described by a combination of several key mechanistic steps: simple bond dissociation, formation of ion‐neutral complex intermediates, hydrogen atom, hydride ion or proton migrations and nucleophilic attack. Selected crucial aspects of these elementary reactions, occurring inside positively charged ions, will be recalled and illustrated by examples taken in recent mass spectrometry literature. Emphasis will be given on the protonation process and its consequence in terms of structure and energetic. Copyright © 2013 John Wiley & Sons, Ltd.     

Stereospecific control of peptide gas‐phase ion chemistry with cis and trans cyclo ornithine residues

We report non‐chiral amino acid residues cis‐ and trans‐1,4‐diaminocyclohexane‐1‐carboxylic acid (cyclo‐ornithine, cO) that exhibit unprecedented stereospecific control of backbone dissociations of singly charged peptide cations and hydrogen‐rich cation radicals produced by electron‐transfer dissociation. Upon collision‐induced dissociation (CID) in the slow heating regime, peptide cations containing trans‐cO residues undergo facile backbone cleavages of amide bonds C‐terminal to trans‐cO. By contrast, peptides with cis‐cO residues undergo dissociations at several amide bonds along the peptide ion backbone. Diastereoisomeric cO‐containing peptides thus provide remarkably distinct tandem mass spectra. The stereospecific effect in CID of the trans‐cO residue is explained by syn‐facially directed proton transfer from the 4‐ammonium group at cO to the C‐terminal amide followed by neighboring group participation in the cleavage of the CO―NH bond, analogous to the aspartic acid and ornithine effects. Backbone dissociations of diastereoisomeric cO‐containing peptide ions generate distinct [b_n]⁺‐type fragment ions that were characterized by CID‐MS³ spectra. Stereospecific control is also reported for electron‐transfer dissociation of cis‐ and trans‐cO containing doubly charged peptide ions. The stereospecific effect upon electron transfer is related to the different conformations of doubly charged peptide ions that affect the electron attachment sites and ensuing N―C_α bond dissociations.     

Au/CeO2催化剂上CO2选择加氢为CO反应及其中间物种研究

CO2的化学转化具有环境及科学双重研究意义.CO2具有很高的化学稳定性,加氢还原是一种有效的转化途径.其中将CO2选择性还原为CO,即逆水汽变换(RWGS)反应(CO2+H2→CO+H2O),具有重要的理论意义和应用价值:(1)CO作为合成气的重要原料,可以通过F-T合成生产更有价值的液体燃料;(2)H2可通过可再生能源电解水制取,实现了全过程的零排放碳循环利用.从热力学角度分析,RWGS反应是一个吸热反应,高温有利于平衡转化率的提高.从动力学角度,一个对正反应有活性的催化剂可同时催化逆反应进行.可还原性载体负载贵金属催化剂,如Pt/CeO2,Au/FeOx,Au/CeO2等,具有很好的低温WGS催化活性,但它们在RWGS反应上的研究较少.我们制备了CeO2负载纳米Au催化剂(HRTEM表征结果表明金高度分散于CeO2载体表面,粒径为4-5 nm),其在常压CO2加氢还原为CO反应中表现出优异的低温活性,分别在450℃,CO2/H2=1,WHSV=12000 mL/(h·g),及400℃,H2/CO2=1,WHSV=6000 mL/(h·g)条件下,CO2转化率接近平衡转化率,且CO的选择性为100%.随着H2/CO2比例增加,CO2转化率明显提高,且维持H2/CO2为1的化学计量比反应.通过原位漫反射红外光谱与质谱相结合的技术,研究了Au/CeO2催化剂上的RWGS反应路径:Au/CeO2催化剂表面形成了甲酸盐中间物种,它的消耗伴随着CO和H2O产物的生成.说明Au/CeO2催化剂遵循中间体机理,这应该是其具有优异低温RWGS反应性能的微观机制.     

Covalent and non‐covalent binding in the ion/ion charge inversion of peptide cations with benzene‐disulfonic acid anions

Protonated angiotensin II and protonated leucine enkephalin‐based peptides, which included YGGFL, YGGFLF, YGGFLH, YGGFLK and YGGFLR, were subjected to ion/ion reactions with the doubly deprotonated reagents 4‐formyl‐1,3‐benzenedisulfonic acid (FBDSA) and 1,3‐benzenedisulfonic acid (BDSA). The major product of the ion/ion reaction is a negatively charged complex of the peptide and reagent. Following dehydration of [M + FBDSA‐H]^? via collisional‐induced dissociation (CID), angiotensin II (DRVYIHPF) showed evidence for two product populations, one in which a covalent modification has taken place and one in which an electrostatic modification has occurred (i.e. no covalent bond formation). A series of studies with model systems confirmed that strong non‐covalent binding of the FBDSA reagent can occur with subsequent ion trap CID resulting in dehydration unrelated to the adduct. Ion trap CID of the dehydration product can result in cleavage of amide bonds in competition with loss of the FBDSA adduct. This scenario is most likely for electrostatically bound complexes in which the peptide contains both an arginine residue and one or more carboxyl groups. Otherwise, loss of the reagent species from the complex, either as an anion or as a neutral species, is the dominant process for electrostatically bound complexes. The results reported here shed new light on the nature of non‐covalent interactions in gas phase complexes of peptide ions that can be used in the rationale design of reagent ions for specific ion/ion reaction applications. Copyright © 2012 John Wiley & Sons, Ltd.     

气相中双取代苯在丙酮离子体系中的加合反应研究

研究了11双取代苯在丙酮离子体系中的离子－分子反应特性及加合离子的碎裂反应特性,发现推电子基有利于加合反应的发生而吸电子基不利于加合反应的发生。双取代异构体的加合产物相对强度可以反映出它们取代基的位置及其性质的差异。邻苯二胺与乙酰基离子形成的加合离子在碎裂反应过程中可以发生与液相中胺的还原烷基化反应相类似的碎裂反应。     

Computational study on the mechanism for the gas‐phase reaction of dimethyl disulfide with OH

The mechanisms for the reaction of CH₃SSCH₃ with OH radical are investigated at the QCISD(T)/6‐311++G(d,p)//B3LYP/6‐311++G(d,p) level of theory. Five channels have been obtained and six transition state structures have been located for the title reaction. The initial association between CH₃SSCH₃ and OH, which forms two low‐energy adducts named as CH₃S(OH)SCH₃ (IM1 and IM2), is confirmed to be a barrierless process, The S? S bond rupture and H? S bond formation of IM1 lead to the products P1(CH₃SH + CH₃SO) with a barrier height of 40.00 kJ mol^?1. The reaction energy of Path 1 is ?74.04 kJ mol^?1. P1 is the most abundant in view of both thermodynamics and dynamics. In addition, IMs can lead to the products P2 (CH₃S + CH₃SOH), P3 (H₂O + CH₂S + CH₃S), P4 (CH₃ + CH₃SSOH), and P5 (CH₄ + CH₃SSO) by addition‐elimination or hydrogen abstraction mechanism. All products are thermodynamically favorable except for P4 (CH₃ + CH₃SSOH). The reaction energies of Path 2, Path 3, Path 4, and Path 5 are ?28.42, ?46.90, 28.03, and ?89.47 kJ mol^?1, respectively. Path 5 is the least favorable channel despite its largest exothermicity (?89.47 kJ mol^?1) because this process must undergo two barriers of TS5 (109.0 kJ mol^?1) and TS6 (25.49 kJ mol^?1). Hopefully, the results presented in this study may provide helpful information on deep insight into the reaction mechanism. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Probing the reactivity of microhydrated α‐nucleophile in the anionic gas‐phase SN2 reaction

To probe the kinetic performance of microsolvated α‐nucleophile, the G2(+)_M calculations were carried out for the gas‐phase S_N2 reactions of monohydrated and dihydrated α‐oxy‐nucleophiles XO^?(H₂O)_{n = 1,2} (X = HO, CH₃O, F, Cl, Br), and α‐sulfur‐nucleophile, HSS^?(H₂O)_{n = 1,2}, toward CH₃Cl. We compared the reactivities of hydrated α‐nucleophiles to those of hydrated normal nucleophiles. Our calculations show that the α‐effect of monohydrated and dihydrated α‐oxy‐nucleophiles will become weaker than those of unhydrated ones if we apply a plot of activation barrier as a function of anion basicity. Whereas the enhanced reactivity of monohydrated and dihydrated ROO^? (R = H, Me) could be observed if compared them with the specific normal nucleophiles, RO^? (R = H, Me). This phenomena can not be seen in the comparisons of XO^?(H₂O)_{n = 1,2} (X = F, Cl, Br) with ClC₂H₄O^?(H₂O)_{n = 1,2}, a normal nucleophile with similar gas basicity to XO^?(H₂O)_{n = 1,2}. These results have been carefully analyzed by natural bond orbital theory and activation strain model. Meanwhile, the relationships between activation barriers with reaction energies and the ionization energies of α‐nucleophile are also discussed. © 2015 Wiley Periodicals, Inc.     

Ion/molecule reactions in a miniature RIT mass spectrometer

Ion/molecule reactions were explored in a newly developed miniature mass spectrometer fitted with a rectilinear ion trap (RIT) mass analyzer. The tandem mass spectrometry performance of this instrument is demonstrated using collision induced dissociation (CID) and ion/molecule reactions. The latter includes Eberlin transacetalization reactions and electrophilic additions. Selective detection of the chemical warfare simulant dimethyl methyl phosphonate (DMMP) was achieved through selective Eberlin reactions of its characteristic phosphonium fragment ion CH3OP(+)(O)CH3 (m/z 93), with 1,4-dioxane or 1,3-dioxolane. Efficient adduct formation as a result of electrophilic attack by the phosphonium ion on various nucleophilic reagents, including 1,1,3,3-tetramethyl urea, methanesulfonic acid methyl ester, dimethyl sulfoxide and methyl salicylate, was also observed using the RIT device. The product ions of these reactions were analyzed using CID and the characteristic fragmentation patterns of the ionic addition products were recorded using multiple-stage experiments in the miniature RIT instrument. This study clearly demonstrates that a small, home-built, miniature RIT mass spectrometer can be used to perform analytically useful ion/molecule reactions and also that instruments like this have the potential to provide a portable platform for in situ detection of organophosphorus esters and related compounds with high specificity using tandem mass spectrometry.     

The reaction of acrolein with acetylperoxyl radicals in the gas‐phase

A rate constant for the epoxidation of acrolein by acetylperoxyl radicals has been determined to be k₄ = (1.3 ± 0.9) × 10⁴ dm³mol⁻¹s⁻¹ at 383 K, which is anomalously fast in comparison with the epoxidation of alkenes. Abstraction of the acyl hydrogen atom from acrolein by acetylperoxyl radicals at 393 K was found to be at least 60 times slower than from acetaldehyde and at least three orders of magnitude slower than abstraction of the acyl hydrogen atom of the epoxide of acrolein. The fast rate for epoxidation of acrolein and the slow rate for hydrogen abstraction provide an explanation for the anomalously slow rate for the autoxidation of acrolein and suggests that acrolein formed during the autoxidation of alkene will react further to give its epoxide, and not exclusively by abstraction of the acyl hydrogen atom as was previously accepted. © 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 277–282, 1999     

Negative charge induced dissociation: fragmentation of deprotonated N‐benzylidene‐2‐hydroxylanilines in electrospray ionization mass spectrometry

Unimolecular reactivities of different N‐benzylidene‐2‐hydroxylaniline anions were investigated in gas phase by electrospray ionization tandem mass spectrometry. All the collision‐induced dissociation spectra of N‐benzylidene‐2‐hydroxylaniline anions show similar ions at phenyl anions, neutral loss of benzonitrile and benzoxazole anions, respectively. The possible fragmentation pathway was probed through deuterium labeling and various group substituents experiments. Computational results were applied to shed light on the mechanism of fragmentation patterns. The proton in the CH=N is reactive in the formation of the concerned ions. Its direct transfer to the oxygen results in 2‐hydroxyphenyl anion. Proton abstraction between benzoxazole and phenyl anion leads to the formation of benzene and benzoxazole anion. Copyright © 2014 John Wiley & Sons, Ltd.     

Copolymerization of carbon dioxide with cyclohexene oxide catalyzed by bimetallic dysprosium complexes containing hydrazine‐functionalized Schiff‐base derivatives

A series of dinuclear Dy^III acetate complexes containing three different hydrazine‐functionalized Schiff‐base ligands ( hmb , hmi, and hb ) have been synthesized by one‐pot reaction with Dy(OAc)₃·4H₂O as the metal precursor. [Dy₂( hmb )₂(OAc)₄]·MeCN ( 1 ·MeCN) and [Dy₂( hmi )₂(OAc)₂(MeOH)₂]·H₂O ( 2 ·H₂O) with keto and enol forms of the corresponding ligands, respectively, were shown the similar core structures but different ratio of Dy^III to OAc^–. Moreover, the different coordination environments of complex [Dy₂( hb )₂(μ‐OAc)₂(OAc)₂(H₂O)₂]·DMF·H₂O ( 3 ·DMF·H₂O) also offered an opportunity to understand the relationship between structural model and catalytic properties. Bimetallic dysprosium complexes 1 – 3 were demonstrated to be active catalysts for copolymerization of carbon dioxide (CO₂) and cyclohexene oxide (CHO) without cocatalysts. To the best of our knowledge, well‐defined catalyst 2 appears to be the first example of an air‐stable bimetallic dysprosium complex that is effective for CO₂/CHO copolymerization and the formation of the perfectly alternating poly(cyclohexenecarbonate) with a high molecular weight. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 321–328     

Radical polymerizations of a silicone‐containing acrylic monomer in supercritical carbon dioxide

Radical homopolymerizations and copolymerizations of 3‐[tris(trimethylsilyloxy)silyl]propyl methacrylate (SiMA) in supercritical CO₂ were investigated. The homopolymer was obtained in CO₂ with a good yield. It was essentially insoluble in pure CO₂ at less than 500 bar at 65 °C but was soluble in a mixture of CO₂ and its monomer (10 w/v %) at 352 bar. The copolymerizations of SiMA with methyl methacrylate, 1,1‐dihydroperfluorooctyl methacrylate, and styrene with various monomer feed ratios were also examined in supercritical CO₂ and in bulk, and the reactivity ratios were determined. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3100–3105, 2000